1. Field of the Invention
The invention relates to indicators and more particularly to those indicators in which a member, normally in an inoperative position, moves in a linear path to an operative position when the input representative of a parameter of a system reaches a predetermined level, and produces an output indicative of the predetermined level.
2. Brief Review of the Prior Art
One example of such an indicator is a differential pressure indicator in which the member moves from the inoperative to an operative position when a difference in pressure between fluid from two sources reaches a predetermined level. In such movement, the member can produce a visual/electrical output. An example of a previous proposal for such a differential pressure indicator is shown in FIG. 1 which is a cross-sectional view of a differential pressure indicator.
Referring to FIG. 1, the differential pressure indicator comprises a diaphragm housing 10, a switch housing 11 and an inner support 12 held between the housings 10,11.
The inner support 12 is provided with an annular wall 13 which defines a recess 14 closed at one end and open at the other. A generally cylindrical magnet 15 is received in the recess 14 and is carried by a mounting 16. An end of the mounting 16 adjacent the open end of the recess 14 is connected to a retainer 17 by a screw 18 with an inner edge of an annular diaphragm 19 being clamped between the mounting 16 and the retainer 17. The outer edge of the diaphragm 19 is clamped between an end of a sleeve 20 extending around the open end of the wall 13 and a radially extending disc 21 clamped between the end of the wall 13 and the sleeve 20.
The diaphragm 19 divides the recess 14 into first and second chambers 22,23. The first chamber is connected to a first inlet and a second chamber 23 is connected to a asecond inlet.
A coil spring 24 is arranged around the mounting 16 between a flange 25 at an end of the mounting 16 adjacent the closed end of the recess 14, and the disc 21. In the position shown in FIG. 1, the coil spring 24 is under compression.
The inner support 12 is provided with an annular projection 26 coaxial with the axes of the recess 14 and the magnet 15 but extending in an opposite direction into the switch housing 11. An indicator button 27 having a cylindrical outer surface 28 is mounted for sliding movement in a cylindrical passage 29 in the switch housing 11 and carries at its inner end a generally cylindrical magnet 30 whose end, in the position of the button 27 shown in FIG. 1, extends into the projection of the inner support 12. Also in this position, an open end 31 of the button 27 extends over the projection 26 with a coil spring 32 held under compression between the end of the button 27 and the inner support 12. The button 27 is retained in this position by magnetic attraction between the two magnets 15,30 which are arranged with opposite poles adjacent. Also in this position, the outer end 33 of the button is generally level with the outer end of the Passage 29 in the switch housing. This open outer end 33 of the passage 29 is covered by a flexible cover 34.
The switch housing 11 also carries a switch 35 extending generally parallel to spaced from the button 27. The switch 35 is connected via cables 36 to a connector 37.
In use, the first and second chambers 22,23 are connected via their inlets to respective sources of fluid under pressure and thus the fluids are passed to the respective chambers, 22,23. When the difference between the two pressures reaches a predetermined level, set by the characteristics of the magnets 15,30, coil spring 24 and the diaphragm 19, the pressure difference on the diaphragm 19 moves the mounting 16 from the position shown in Figure 1 outwardly of the recess 14. An overtravel stop is provided when the container 17 abuts the diaphragm housing 10. This movement moves the magnet 15 away from the closed end of the recess 14 and so breaks the magnetic attraction between that magnet 15 and the magnet 30 in the switch housing 11. Accordingly, the coil spring 32 associated with the button 27 moves the button 27 outwardly of the passage 29 so that the end 33 of the button 27 projects from the passage 29 into the cover 34. This provides a visual indication that the predetermined pressure difference has been exceeded.
This movement also actuates the switch 35 to pass an electrical signal through the cable 36 to the connector 37 from which it can be taken to an electrical indicator.
The switch housing 11 and the button 27 are commonly made out of aluminium or aluminium alloys. These parts are permanently in contact where the button 27 passes through the passage 29. In general, these materials give adequate performance and reliability characteristics except in a severe environment such as encountered on an aircraft or other mechanism which can induce damaging vibration forces. Under the influence of such forces, the button 27 can be moved in all three planes and can also spin on its axis giving frictional movement on its cylindrical outer surface and on its circular end face. This fretting can cause the formation of debris containing, for example, iron, aluminum and aluminium oxides. The vibration encountered after significant debris has been generated is often increasingly damaging as the debris acts as a grinding grit-type medium.
It has been found that, in such severe environments, the button 27 can be caused to jam on release of the force of the first magnet 15 and so fail to give a proper indication.